George frederick wynne



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G. P. WYNNE. PHOTOGRAPHIG EXPOSURE METER. No. 557,246.

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UNITED STATES PATENT ()FFICE.

GEORGE FREDERICK \VYNNE, OF MINERA, ENGLAND.

PHOTOGRAPHlC-EXP OSURE METER.

SPECIFICATION formingpart of Letters Patent No. 557,246, dated March 31,1896.

Application filed January 25,1894. Serial No. 498,051. (N0 modeI.)

To aZZ whom it may concern:

Be it known that I, GEORGE FREDERICK IVYNNE, a subject of the Queen of Great Britain, residing at Minera, near \Vrexham, in the county of Denbigh, in the Principality of IVales, England, have invented certain new and useful Improvements in Apparatus for Calculating Photographic Exposures, of which the following is a specification.

This invention relates to an improved apparatus for calculating photographic exposures of that class in which the light is first tested by an actinoineter in the usual manner and the photographic exposure deduced therefrom.

The duration of exposure required for a photographic plate is affected by four conditions, namely: first, the actinic value of the light; second, the character of the subject; third, the ratio of the diaphragm of the lens to its focal length, and, fourth, the sensitiveness of the plate used. In other forms of calculating instruments hitherto used these four factors have been calculated separately, and a specially divided circular slide-rule (with pointers or with a combination of stops and pointers) has been provided for each factor, and when the resulting exposure has been shown by the instrument it has only given the one exposure for the particular assumed values of the factors.

My improved calculating instrument is preferably made in two forms, set forth in the accompanying drawings.

Figure 1 shows plans of my apparatus arranged in a cylindrical box or watchcase form; Fig. 2, a section of same; Fig. 3, a plan of disk; Figs. 4 and 5, two slightly-varying forms of the slide-rule form.

In Figs. 1, 2, and 3 the lid or glass F, upon which the diaphragms and plate'speed-numbers scale G is printed, is made so that it can be turned around upon the body of the box J by means of a milled edge j or other suitable device. The periphery of the box and lid or the part near the edge of the glass and the outer edge of the exposed portion of the body of the box, as shown in Fig. 1,, are each divided, geometrically, into two scales, as shown-namely, a scale G of diaphragm numbers of plates employed, as hereinafter explained, which are ascertained from actual takes to darken to a standard tint of disk or part a. D is a circular box having a rotatable upper part or lid J with an opening A provided with a disk or one or more strips of standard tint of any suitable shape and size a, and leaving exposed a part B, through which a small segment of the rotatable disk of sensitized paper 0, hereinafter described, can be seen.

The box is furnished with spare disks E, of sensitized paper, and these disks can be prevented from rotating in the box (when the lid is turned) by being provided with lips I engaging in slots in the box or otherwise, the upper part of the box being rotated to expose fresh surfaces to the actinic rays of light. Instead, however, of these lips or slots I may use any other suitable device for fulfilling the same object-for instance, by fixing the disk to the box by a suitable cement or by placing it in contact with such a surface attached to the box that the friction of this surface acting upon the disk is sufficient to turn it, or in any other suitable way.

The disk of sensitive paper may be kept pressed up to the opening in the lid by any suitable device; but I preferably employ a disk of thick felt D, which possesses a con siderable amount of elasticity and exerts sufficient friction to turn the disk of sensitive paper, especially if lips or projections I are formed upon it, as before described.

0 is the disk of sensitive paper, (shown in plan, Fig. 3;) D, the felt pad, and E a series of spare disks to be used in succession in place of O; L, a pin holding the pad from turning; K, a plate inclosing between it and box D a space for spare disks; M, milled edge to box D.

F is the glass, to which is securely cemented the-diaphragmnumber-scale, as shown; J, upper part of body; D, lower part of body, having milled edge for turning the same or for holding it while the top is turned. It will thus be seen that by moving the glass relatively to the upper part of the body the scales are adjusted to each other, while by moving the lower part around slightly a fresh piece of the sensitized paper disk is exposed.

It is obvious that instead of having one of the scales fixed to the glass it might be fixed to an extra and specially made part of the case; but the plan shown being the simplest is the one I prefer to employ.

Fig. 4 shows a cheaper form of the apparatus, but not so convenient, in the form of a slide-rule. In this the aetinometer time is placed on the slide, and the diaphragm and plate-speeds on the rule. Fig. 5 is a similar rule, but with the actual relative speeds of plates set forth in another column on the rule. The actinometer is in a box behind (shown in dotted lines) and is, preferably, of the type set out in Figs. 1 and 2.

In all the figures the speed of the plate, instead of being expressed by an arbitrary number, may be and is in the drawings expressed by the size of diaphragm through which a plate of that particular speed would require the actinometer time for its correct exposure upon a normal subject. The more rapid the speed of the plate the smaller would be the size of its corresponding diaphragm, and vice versa.

\Vhere the scale of relative or comparative speeds is used, the unit of plate-speed, or speed No. 1, may be calculated as the speed which takes (for a normal subject) the same time for exposure of a photographic plate of this rapidity through a standard diaphragm (the diameter of which is equal to the focal length of the lens divided by four) that the sensitive paper in the actinometer takes to color to a standard tint. It therefore follows that the comparative speeds of any plates of different diaphragm numbers are represented by the uniform standard numbers, (of the Photographic Society of Great Britain, which correspond with the F numbers of the plates compared.

The unit might obviously be calculated with reference to anyother diaphragm, and if the unit is taken with reference to F 45 then when the scales of the instrument are placed in their normal position, as shown, the comparative or relative speed of any plate will be simultaneously found against its diaphragm numberthus, F 45:1, F 64:2, F 4, F 2S:*, F 8 and so on.

If F 4 is taken as the unit for the comparative speeds of the plates, the figure which represents the speed of the plate will also represent the ratio of plate exposure to actinometer exposure for this standard diaphragm called F 4. It is obvious that the uniform standard numbers or any other comparative numbers might be engraved upon the scales against the corresponding diaphragm numbers, as shown in one plan of the slide rule form of instrument, in order to show the actual relative speeds of the plates; but as doseconds against F 45.

ing so would not in any way affect the resulting calculation and would make the instrument more complicated it is preferred to use only the F or diaphragm numbers.

The diaphragms on the scale of diaphragms are graduated from the largest to the smallest in exactly the same proportion as the timescale and in such manner that when the instrument is set correctly for one diaphragm it is at the sametime set correctly for all diaphragms from the largest to the smallest. To calculate an exposure, therefore, fora normal subject, it is only necessary to set the actinometer time on exposure-scale against the diaphragm number of the particular plate used, when against each diaphragm will be found on the same scale the correct exposure for that particular stop.

For exceptional subjects, which require a longer or shorter exposure than the normal, instead of having a separate scale, as hitherto employed for calculating the variation from the normal exposure due to the subject, it is only necessary to move the slide a certain number of divisions higher or lower than the normal position, according to the amount of variation required, and calculate as at this higher or lower figure. It will thus be seen that by the particular construction of the instrument and arrangement of scales this one scale of diaphragms represents in itself three factors of exposurenamely, the sizes of diaphragms, the speed of the plate, and the variation of exposure due to special subjects and that the exposure-scale also represents both the actinometer time and also the exposure in seconds.

It will be seen that the scales can be divided either upon the periphery of a box and lid, upon two movable concentric rings or disks, or upon a rule and slide.

The mode of working is as follows: Instead of expressing the sensitiveness of a plate by an arbitrary number it is expressed by the size of diaphragm through which the plate would require the actinometer-time for its correct exposure upon a normal subject. Thus Ilford ordinary plates, No. F 39, means that an ordinary subject upon these plates, through stop F 39, would require the actinometer-time for its correct exposure.

To calculate the correct exposure, turn the movable scale of the instrument until the actinometer-time in seconds upon the exposurescale is opposite the diaphragm number of the plate. Then the correct exposure in sec ends and fractions of seconds will be found simultaneously against each stop from the largest to the smallest.

Example No. 1: Plate No. F 45; actinometer, twelve seconds; stop, F 16. Put twelve Then against F 16 will be found one and one-half seconds, which is the correct exposure, and also against every other stop the correct exposure for that particular diaphragm.

Example No. 2: Plate No. F 90; actinome- IIO ter, three seconds; stop, F 5.6. Put three seconds againstF 90. Then againstF 5. 6 will be found one eighty-fifths of a second, and also against every other stop the correct relative exposure. If in an interior exposure or an exposure in a shaded situation, the sensitive paper takes minutes instead of seconds to color to the standard tint, the figures upon the exposure-scale may be read as minutes instead of seconds.

Photographing exceptional subjects: For panoramic views or open landscape with no dark objects in the foreground instead of setting the actinometer-time against the actual plate-speed number set it two speeds higher than this and calculate as if the platespeed were this higher figure. Thus with plate-speed F 45 set actinometer-time to F (54;, or with plate-speed F 78 set it to F 111.

For sea and sky set the actinometer-time six speeds higher than the actual plate-speed used and calculate as if plate-speed were this higher figure. Thus with plate-speed F 15 set it to F 128, or with plate-speed F 90 set it to F 256. If preferred, the exposure for sea and sky may be found by calculating as for an ordinary exposure and dividing by from 8 to 10.

It will be seen from the foregoing description that the simplicity of the instrument is not attained by ignoring any of the factors of exposure, but by the scales being so designed that each of them represents in itself two or three of those factors. Thus the scale of diaphragms represents, first, the scale of diaphragms; secondly, the speed of plate, and, thirdly, the variation of exposure due to subject, while the time-scale represents the actinometer-time and also the exposure in seconds or minutes.

The scales are also so divided and arranged that when they are set correctly for one diaphragm they are simultaneously set correctly for every diaphragm, from the largest to the smallest.

Having thus described my invention, what I claim, and desire to secure by Letters Patent, is

1. The combination of the watchcase made in two pieces D and J rotatable on each other and cover F, also rotatable on part J with the two scales G and H and the sensitive disk 0, each of the three being practically a fixture on and rotating with its part D J or F.

2. In an exposure-meter, the combination of a watch-back D having milled edge M, watch-body J, pad D, disk 0, standard tintsurface a and aperture B for the purposes described.

3. The combination of the box D, space therein for holding spare disks E, plate K, felt pad D, disk 0 and lid A having actinometer eccentric orifice B substantially as described.

4. In an actinometer, the combination of a sensitized disk 0, means for inclosing and rotating same relatively to the cover, with a cover having an eccentric orifice and adjacent standard colored part substantially as described.

5. The combination of case D, fiat interior face K, pad D, anchoring device L, with the disk 0 and perforated dial plate substantially as described.

6. The combination of a scale of diaphragms and a scale of exposures so divided and marked that, in whatsoever position the scales are placed relatively to the other, if the exposure opposite one diaphragm be correct for that diaphragm, the exposure opposite every other diaphragm will be correct for such diaphragm, substantially as described.

In testimony whereof I have signed my name to this specification in the presence of two subscribing witnesses.

GEORGE FREDERICK WVYNNE.

Witnesses:

HUGH J. HOWELL EVANS, H. M. BROOKFIELD DAVIS. 

